Capacitively coupled machine tool safety having a self-test network

ABSTRACT

A machine tool safety system includes an electric field generator capacitively coupled to a receptor antenna. A self-test network includes a self-test radiating antenna and a circuit arrangement operative to apply a test signal to the self-test antenna. The test signal may be of sufficient strength to induce in the receptor antenna a first excitation signal having a magnitude greater than a first predetermined threshold to verify that the safety system is operative to detect the entry of the field generator carried on an operator&#39;s person into a hazard zone proximal to the machine tool. The test signal may be of sufficient strength to induce a second excitation signal greater than a second threshold to verify that the safety system is operative to detect the entry of the field generator into an arming zone spaced from the hazard zone.

CROSS REFERENCE TO RELATED APPLICATIONS

Subject matter disclosed herein is disclosed and claimed in thefollowing applications filed contemporaneously herewith:

Capacitively Coupled Machine Tool Safety System, filed in the name of G.R. Hoffman on Feb. 2, 1982, and accorded Ser. No. 345,193; and A DigitalQuantizer, filed in the name of H. E. Betsill on Feb. 2, 1982 andaccorded Ser. No. 345,192.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a safety system which utilizes a capacitivelycoupled transmission arrangement to provide a uniform pattern in apredetermined sensitivity zone around the machine tool and in particularto a safety system having a self-test capability.

2. Description of the Prior Art

Power driven machine tools constitute a primary cause of industrialaccidents through crippling or severance of the hands or fingers of themachine tool operator. One attempted solution to this problem is theutilization of mechanical barriers to shield especially dangerous areason the machine tool and prevent the introduction of any portion of theoperator's anatomy into those areas of the tool while still allowingentry of the workpiece. For example, in the case of a pipe flangingtool, a barrier plate having an aperture sized to generally match theoutside diameter of a tubular workpiece may be used to prevent theinsertion of the operator's fingers or thumb into proximity with thejaws of the flanging machine. To be more fully certain that theoperator's hands are protected, it is necessary that the apertureclosely match the outside diameter of the workpiece. However, theaperture in the barrier plate must not be so restricted as to preventthe insertion of various sized or odd-shaped workpieces into the machinetool. But if the aperture is sized to receive a wide range of workpiecesizes, the possibility exists that a portion of the operator's body maybecome engaged by the jaws. It is difficult to find a barrier plateguard arrangement which is flexible enough to permit entry of a varietyof workpiece sizes and shapes, yet which simultaneously affordsprotection to the operator and does not appreciably diminish thethroughput of the machine tool.

Other mechanical expedients used in the art include the provision ofbarrier bars or touch bars which extend across the width of a machinetool. When struck by a portion of the operator's body these elementsgenerate a signal which disables the machine tool. For example, in thecase of the power rollers used to work rubber or elastomers, theoperator stands on a platform facing two large counterrotating rollerswhich work the elastomeric material. Safety trip bars at knee level,elbow level and, perhaps even a head level trip wire, extend parallel tothe axis of the rollers and interpose themselves between the body of theoperator and the rollers. Thus if the operator becomes in any waysnagged or drawn into the rotating machinery, the likelihood is that aportion of his body would engage against one of the trip members andwould thereby disable the machine tool. However, it is possible that therestraints may be missed or, perhaps more likely, the restraints wouldterminate the operation of the machine tool only after injury has beeninflicted upon the operator.

A number of radio controlled machine tool protection systems areavailable. Such systems include those described in the following U.S.Pat. Nos.: 4,075,961 (Harris); 4,057,805 (Dowling, assigned to theassignee of the present invention); 3,983,483 (Pando); 3,950,755(Westbrook, Sr.); 3,896,425 (Erichsen); 3,872,455 (Fuller et al.);3,409,842 (Embling et al.); and 1,847,872 (Hand). Each of these devicesutilizes some variant of an inductively coupled radio frequencytransmitter-receiver arrangement. In such systems the sensitivitypattern of the receiving antenna is dependent upon the orientation ofthe transmitter on the person of the operator and is also subject tonull spaces or voids in coverage. These systems detect and trip ifmaterial must be fed into the hazard zone surrounding the machine tool.

It is believed to be advantageous to provide a machine tool safetysystem which utilizes a capacitively coupled electric field generator(transmitter)-capacitive receptor antenna arrangement to eliminate theexistence of null spaces and voids in the sensitivity coverage whichexist with an inductively coupled transmitter-receiver arrangement. Itis believed to be of further advantage to provide the electric fieldgenerator (transmitter) in a form able to be conveniently carried uponsome portion of the person of the operator, e.g., the wrist. It is alsobelieved advantageous to provide a safety system the sensitivity ofwhich monotonically increases as the distance between the electric fieldgenerator (transmitter) and the capacitive receptor antenna mounted onthe machine tool decreases.

SUMMARY OF THE INVENTION

The present invention relates to a machine tool safety system whichincludes an electric field generator, or transmitter, adapted togenerate an electric field and a capacitive receptor antenna mountableto a machine tool, the capacitive receptor antenna and the generatorcooperating to define a capacitively coupled transmission arrangementthat overcomes the void and null problems associated with inductivelycoupled transmitter-antenna arrangements.

The electric field generator (transmitter) is adapted to be carried onsome portion of the person of the operator, such as the wrist and isadapted to generate an electric, or potential field. The electric fieldgenerator includes a transmitting antenna the field of which can beshaped to overcome body shielding. The electric field generator uses thebody of the operator upon whom it is carried as part of an electricfield radiator.

The capacitive receptor antenna is mountable to the machine tool or in apredetermined location with respect to the machine tool and iscooperable with the electric field generator to form a capacitivelycoupled transmission arrangement able to induce, by the action of theelectric field through the capacitive coupling, a monotonicallyincreasing electrical signal, the magnitude of which is functionallyrelated to the distance between the electric field generator and thecapacitive receptor antenna. The capacitive receptor antenna includes aconductive member configured in a predetermined manner to define apredetermined corresponding sensitivity zone which may generally beprovided in a predetermined shape.

A signal processor is connected to the capacitive receptor antenna andis responsive to the signal induced in the antenna to generate a first,and/or a second indicator when the magnitude of the induced signalexceeds a first, and/or a second predetermined threshold, respectively.Any predetermined number of indicators may be generated. The indicatorsmay be used to form the basis for decisions aimed at safe operation ofthe machine tool.

In the preferred embodiment of the invention the signal processordevelops a representation of the magnitude of the electrical signalinduced in the capacitive receptor antenna for comparison with thepredetermined thresholds. It is also desirable in the preferredembodiment that the field produced by the electric field generator(transmitter) varies in accordance with a predetermined radio frequencycarrier and is, in addition, modulated in accordance with apredetermined modulation characteristic. The field may exhibit apredetermined recurrence rate and duty cycle. The use of a carrierfrequency permits selective high amplification of the induced signal andrejection of local changes in the electric field originating fromextraneous sources. The modulation characteristic applied to the carrierfrequency provides discrimination against the possibility ofinterference from other transmitters operating at the same carrierfrequency (e.g., a local radio station). The signal processor alsocomprises a network for amplifying the carrier frequency and detectingthe modulation characteristic of the induced signal to verify that thesignal is induced in the capacitive receptor antenna by the action ofthe electric field from the electric field generator (transmitter).

The preferred embodiment of the invention also includes a self-testradiating antenna disposed in proximity to the capacitive receptorantenna and a self-test network connected to the self-test radiatingantenna for generating a first and a second excitation signal operativeto respectively induce in the capacitive receptor antenna a test signalhaving a magnitude greater than the first and second thresholds.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription thereof taken in connection with the accompanying drawingswhich form a part of this application and in which:

FIG. 1 is a generalized block diagram of the machine tool safety systemusing a capacitively coupled electric field generator-capacitivereceptor antenna arrangement in accordance with the teachings of thepresent invention in which a discrete logic implementation of a signalprocessor is used;

FIG. 2A is a detailed schematic diagram of an electric field generator(transmitter) used in the present invention; FIG. 2B indicates thewaveforms of signals present at predetermined locations in FIG. 2A; andFIG. 2C is a schematic diagram of an electric field generator useful inconnection with a discrete logic implementation of a signal processor;

FIGS. 3A and 3B are, respectively, isolated perspective views of twospecific configurations of a capacitive receptor antenna of the thepresent invention for use with a pipe flanging apparatus and anelastomer rolling apparatus, respectively, while FIG. 3C is a sectionalview taken along section lines 3C--3C in FIG. 3A;

FIG. 4A is a schematic diagram of a receiver used in the embodiment ofthe safety system of the present invention having a discrete logicsignal processor while FIG. 4B is a graphic depiction of the functionalrelationship between the input signal to the receiver and the outputfrom the receiver;

FIG. 5 is a schematic diagram of a discrete logic implementation of acontrol logic network used in connection with the embodiment of thepresent invention having a discrete logic signal processor;

FIG. 6A is a schematic diagram of a portion of the control logic networkshown in FIG. 5 illustrating the signal identification network and theacceptance network while FIG. 6B indicates the waveforms of signalspresent at predetermined locations in FIG. 6A;

FIG. 7A is a schematic diagram of the self-test network used inconnection with the embodiment of the present invention having adiscrete logic signal processor, while FIG. 7B is a timing diagramindicating the sequence of operations of the network shown in FIG. 7A;

FIG. 8 is a generalized block diagram of a machine tool safety systemusing a capacitively coupled electric field generator-capacitivereceptor antenna arrangement in accordance with the present invention inwhich a microcomputer controlled implementation of a signal processor isused;

FIG. 9 is a schematic diagram of the oscillator-mixer and filter stagesused in the receiver of the embodiment of the present invention having amicrocomputer controlled signal processor;

FIG. 10 is a schematic diagram of one stage of the I.F. amplifier andpeak detector network used in the receiver of the embodiment of thepresent invention having a microcomputer controlled signal processor;

FIG. 11 is a schematic diagram of a self-test transmitter network usedin connection with the embodiment of the present invention having amicrocomputer controlled signal processor;

FIG. 12 is a block diagram of the microcomputer used to control thesignal processor in the embodiment of the present invention shown inFIG. 8;

FIGS. 13A through 13E are timing diagrams and register sequence diagramsillustrating the operation of the microcomputer-controlled signalprocessor in the embodiment of the invention shown in FIG. 8;

FIGS. 14, 15, 16A through 16D and 17 are flow diagrams of the programused in connection with the microcomputer-controlled signal processor inthe embodiment of the invention shown in FIG. 8; and

FIGS. 18A through 18D are a flow diagram of the program used inconnection with the self-testing of the microcomputer-controlled signalprocessor in the embodiment of the invention shown in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the following detailed description similar reference numeralsrefer to similar elements in all figures of the drawings.

FIG. 1 is a generalized functional block diagram of a machine toolsafety system 19 in accordance with the present invention in which adiscrete logic implementation of a signal processor is used. Ageneralized functional block diagram of the machine tool safety system19, which utilizes a microcomputer controlled implementation of thesignal processor (two channel receiver) is shown in FIG. 8. In themicrocomputer-controlled signal processor embodiment generally similarfunctional units are indicated by primed reference numerals.

The machine tool safety system 19 includes an electric field generator,or transmitter, indicated by reference character 20, a capacitivereceptor antenna 21 and a signal processor 23. The signal processor 23itself includes a receiver 24, and a control logic network 26. Thesignal processor 23 is connected to a machine tool control interface 28which interacts with the signal processor 23, the machine tool and theoperator. The interface may include a power relay network which controlsthe application of electric power to the machine tool. The transmitter20 and the capacitive receptor antenna 21 cooperate to define acapacitively coupled transmission system.

The electric field generator 20 is preferably adapted to be carried on aportion of the person of the machine tool operator, as for example thewrist. One or more electric field generators may be used to definemultichannel operation (e.g., FIG. 8), each electric field generatorbeing worn in a different location on the person of the operatordepending upon the situation and the perceived safety risk. For example,when used with a pipe flanging apparatus where only one of theoperator's hands is likely to be exposed to the risk of entry into ahazard zone, it may be sufficient to utilize a single wrist transmitteroperating in one frequency channel. In an elastomer rolling mill whereeither arm may potentially enter a hazard zone, it may be desirable toutilize a dual frequency channel arrangement wherein a transmitter isprovided on each wrist of the operator. Extension to Q frequencychannels of operation and to R operators, of course, lies within thecontemplation of the present invention. As is discussed herein, in thepreferred embodiment of the invention the electric field produced by theelectric field generator 20 is varied in accordance with a predeterminedcarrier frequency and is also modulated in accordance with apredetermined modulation characteristic.

The capacitive receptor antenna 21 is mountable in any predeterminedlocation with respect to the machine tool. By the term "capacitivereceptor antenna" it is meant a device which is sensitive to theelectric field produced by the electric field generator 20 andresponsive thereto to produce an electrical signal output, the "inducedsignal." The capacitive antenna 21 is a device adapted to capacitivelycouple a conductor (antenna) disposed as part of the electric fieldgenerator 20 with a conductive member provided within the capacitivereceptor antenna 21. The capacitive receptor antenna 21 is configured toexhibit a predetermined sensitivity pattern that covers a predeterminedhazard zone about the machine tool, as discussed in connection with thespecific examples shown in FIG. 3. The capacitive receptor antenna 21responds to the electric field generated by the electric field generator20 by producing an induced signal on an output line 22 thatmonotonically increases as the distance between the electric fieldgenerator 20 and the capacitive antenna 21 decreases. In this manner thevoids and nulls associated with electromagnetic and magnetic couplingtechniques used in various prior art safety devices are eliminated.Judicious selection of the configuration of the conductive member inview of the machine tool's configuration and with appropriate conductiveshielding provides the capability of forming unique sensitivity patternswhich may be desirable when protecting various machine tools.

The signal processor 23 is connected to the capacitive receptor antenna21 and responds to the induced signal on the line 22 to generate one orany predetermined number of indicators which may be used to form thebasis of decisions aimed at the safe operation of the machine tool. Theindicators are generated by comparison of the magnitude of the inducedsignal with each of an appropriate predetermined number of thresholds.

The signal processor 23 may be implemented in a discrete logicimplementation as, for example, is shown in FIGS. 1 through 7, or in amicrocomputer controlled implementation as, for example, is shown inFIGS. 8 through 18. Each implementation of the signal processor 23includes a receiver 24 and a control logic network 26.

In the discrete logic implementation, the receiver 24 is connected tothe capacitive antenna 22 and is responsive to the received inducedsignal to generate a first electrical signal (SAFETY LEVEL), and asecond electrical signal (ARM). The receiver 24 is preferably configuredto impart a predetermined functional relationship between the magnitudeof the first and second signals and the distance between the electricfield generator 20 and the capacitive receptor antenna 21. Preferably,the receiver 24 is configured to respond in a more sensitive fashion tochanges in the magnitude of the first signal (SAFETY LEVEL) when theelectric field generator 20 is at distances closer to the capacitivereceptor antenna 21. This provides for greater sensitivity as theelectric field generator 20 (i.e., the portion of the operator's anatomycarrying the electric field generator 20) comes closer to the hazardzone. In addition, the receiver 24 is configured to respond with greatersensitivity to variations in the magnitude of the second signal (ARM) asthe electric field generator 20 is farther away from the capacitivereceptor antenna 21. An exponential form of the relationship isillustrated graphically in FIG. 4B. Of course, the receiver 24 may beconfigured to impart any desired predetermined functional relationship.

The first electrical signal, the SAFETY LEVEL signal, is applied over aline 30 to the control logic 26. When the magnitude of the firstelectrical signal on the line 30 exceeds a predetermined threshold(representative of the entry in the hazard zone of the portion of theperson of the operator carrying the transmitter) a first indicator, aMACHINE DISABLE signal, is applied on a line 32 to the machine toolinterface 28. The machine tool is therefore disabled to avert thepossibility of injury to the operator. The second signal (ARM) is outputon a line 36 from the receiver 24 to the control logic 26. The controllogic 26 contains circuitry adapted to compare the magnitude of thesignal on the line 36 with a predetermined threshold. If the signal onthe line 36 exceeds the threshold (representative of the fact that theoperator has entered a predetermined arming zone with an operativetransmitter) a second indicator, MACHINE ENABLE signal, is applied onthe line 38 to the machine interface 28.

In the preferred embodiment of the invention the field generated by theelectric field generator 20 is varied in accordance with a predeterminedcarrier frequency and the carrier is modulated in accordance with apredetermined modulation characteristic so as to exhibit a predeterminedrecurrence rate and duty cycle. Accordingly both the SAFETY LEVEL andthe ARM signals will vary at a frequency which corresponds to thecarrier frequency and in a manner which corresponds to the modulationcharacteristic. The signal processor 23 contains circuitry whichamplifies the induced signal and detects the modulation characteristicof the first and/or the second signal and compares the detectedmodulation characteristic with a modulation characteristic reference. Atrue comparison is indicative of the fact that the induced signal fromthe capacitive antenna 21 is produced as a result of the transmissionfrom the electric field generator 20 and its antenna 50C. Such a truecomparison may be made a necessary condition before the machine tool isenabled and/or disabled. The presence of a true comparison is preferableas a necessary condition for the generation of the MACHINE ENABLE signalwhich is required for operation of the machine tool. If more than onetransmitter is used, each transmitter operates at a different carrierfrequency. A separate receiver channel (which may be connected to thesame or to a different capacitive antenna) and a separate processorchannel are used for each transmitter carrier frequency.

In the microcomputer controlled implementation of the signal processor23' shown in FIGS. 8 through 18, each channel of the receiver 24'responds to the induced signal to translate it to an IF frequency,develops four successive estimations of the induced signal level, andpeak detects in a sample-and-hold manner under the control of amicrocomputer 460 (FIGS. 8 and 12) in the control logic 26'. The foursuccessive estimations are applied to an analog-to-digital converter andread by the microcomputer to generate one binary numericalrepresentation in log format of the induced signal at the sample time.Thresholds are established by potentiometers 352 and 354 (FIG. 8) whichcan be accessed by the microcomputer and used to develop the indicatorsherein discussed. When the electric field is varied in accordance withthe radio frequency carrier and modulated in accordance with thepredetermined modulation characteristic, an exponentially averagedrepresentation of the signal is developed by computer algorithm. Thevirtue of the microcomputer control permits ease of adaption of thesignal processor 23' to generate indicators needed to meet the safetyrequirements of a specific machine tool. A listing of the program forthe microcomputer is provided in the Appendix attached hereto and herebymade part of this application.

The safety system 19 includes several precautionary features. First, thesafety system 19 includes a self-test generator 42 adapted to applyself-testing signals over a line 44 to a self-test antenna 46 disposedin a predetermined location to the capacitive receptor antenna 21. If aself-test function is used, the satisfactory passing of a sequence ofself-testing operations may be imposed as a further condition to thegeneration of an indicator.

As a second precaution the electric field generator 20 may be providedwith a network which monitors its operation to insure a stable outputsignal level. One manner in which this may be accomplished is to measurethe battery voltage of the electric field generator, and should thesupply voltage be found to be too low, a network is operative toimmediately discharge the supply rather than allow the supply to degradeslowly with further use. Alternatively, if the supply falls below apredetermined threshold, the carrier is no longer modulated at thepredetermined recurrence rate, thus causing the received signal to berejected.

ELECTRIC FIELD GENERATOR

FIG. 2A is a detailed schematic diagram of the electric field generator20 used in connection with the microcomputer controlled signalprocessor. FIG. 2C is a schematic diagram of an electric field generatorwhich may be used in connection with the discrete logic implementationof the signal processor.

The electric field generator 20 is a battery operated crystal controlleddevice preferably sized to be mounted within a conductive, metalliccasing 50 fabricated from e.g., stainless steel or aluminum. The casing50 is about the size of a wrist watch. The watch size is believed mostconvenient for carrying by a machine operator on the wrist. Of coursethe electric field generator 20 may be modified for carrying about anypredetermined portion of the anatomy of the operator.

The crystal controlled oscillator contained within the electric fieldgenerator 20 must be referenced to ground on one terminal in order toestablish the electrical field produced thereby. Accordingly, acapacitive or ohmic contact between a portion of the casing and the bodyof the person is required. The cover and metal band 50C of the casing 50is insulated from the bottom 50B by a layer of insulating material 50L.The bottom 50B of the casing 50 is in contact with the skin of theoperator. The band is suspended above the skin of the operator by aninsulating pad 50P such that a potential field is established betweenthe skin of the operator and the band. This provides nearly isotropiccoverage with minimum shielding effects.

Alternatively, the electric field generator 20 may comprise twoconcentric bands separated by an insulator such that both bands aredriven in opposition. The entire assembly is suitably insulated on allsurfaces to prevent body contact with the operator and to protect theelectric field generator from the environment.

In some instances, as in the elastomer rolling environment shown in FIG.3B, it may also be desirable that the operator occupy a position on aground plane so that the body of the operator is capacitively coupled toground to further stabilize the strength of the field produced by theelectric field generator 20. It should be noted that in those instanceswhere the operator is manipulating large metallic workpieces, as inconjunction with a pipe flanging machine, the radiated field from theelectric field generator 20 increases when the metallic workpiece isgrasped before the workpiece contacts the machine. Since this occurrencehas a tendency to increase the induced signal, it is viewed as asubstantially failsafe condition.

Each battery powered electric field generator is preferably configuredto emit a unique modulated signal at a frequency sufficiently low toprovide essentially isotropic coverage of the hazard zone and the armingzone (FIG. 3) of the capacitive receptor antenna. If a second electricfield generator is utilized, the second transmitter emits a secondsimilarly modulated signal at a different carrier frequency spaced fromthe first carrier frequency to a duplicate second receiver which may beconnected to the same or a different capacitive receptor antenna. Thesignal from the second transmitter may be modulated at the same or adifferent modulation characteristic.

In view of the fact that the signal processor 23 defines basically anamplitude sensing and discrimination system, any degradation of eitherthe receiver system gain or electric field generator power will causethe hazard zone limit to move closer to the machine tool. To preventsuch a condition from arising without warning, the periodicallyoperating calibrated self-test generator system 42 operates through theself-test antenna 46 to insure that the receiver gain does not degrade.The self-test system is discussed herein in connection with FIGS. 7 and11. At the same time, degradation of the strength of the electric fieldis prevented by means of a continuously operating circuit monitor whicheffectively drains the power source, typically a battery, to preventpartial recovery of the battery during periods of disuse or whichdisables the modulator to permit transmission of only an unmodulatedcarrier, which condition would result in rejection of the signal by thesignal processor.

With reference to FIG. 2A, the detailed schematic diagram of theelectric field generator 20 used in the operator safety system of thepresent invention having the microcomputer controlled implementation ofthe signal processor is shown. The transmitter 20 is operated by asingle cell mercury battery 54 of the hearing aid type. The battery 54is preferably that manufactured and sold by Everready Inc. under modelnumber EP675E6. The transmitter is operated upon closure of a switch 56by the machine operator. The switch 56 is conveniently mounted on asuitable location in the casing 50. The expected lifetime of the battery54 is approximately one hundred hours at which time the voltage outputdrops from the nominal fresh value of 1.35 V to 1.2 V.

As seen in FIG. 2A the electric field generator 20 includes a radiofrequency oscillator network 58, a radio frequency amplifier modulatornetwork 60, an output parallel resonant network 62 (69,70), a modulatingnetwork 64 and a battery monitor network 68. The radio frequencyoscillations are generated by the transistor 58A in combination with acrystal 66 and associated components. Suitable for use as thetransistors 58A and 60A are devices manufactured by Motorola and soldunder model number 2N2222A.

The output coil 69 and shunt capacitor 70 steps up the output voltage.In practice, about six volts peak-to-peak is available at the output.The radio frequency carrier preferably lies in the range from 1.5 to 1.9MHz. This signal is present at the test point TP-1 as illustrated inFIG. 2B. Depending upon the carrier frequency, the magnitude of thecapacitor 70 is either 300 pF. (for 1.5 to 1.7 MHz operation) or 220 pF.(for 1.7 to 1.9 MHz operation).

The RF carrier is switched on and off by the modulating network 64. Anoperational amplifier 64A such as that sold by National Semiconductorunder model number LM10 is used to produce a square wave voltagereference. The recurrence rate is about seventy Hertz, although anypredetermined recurrence rate may be used. Proper setting for themodulator 64 is established by a potentiometer 64R which is adjustableto provide a square wave signal of a predetermined duty cycle(preferably fifty percent) to the amplifier 60. Of course, any suitableduty cycle may be employed. This waveform, derived from test point TP-2,is illustrated in FIG. 2B. The carrier signal, modulated in accordancewith the modulation characteristic imparted by the modulator 64,produces an output waveform radiated from the antenna 50C of the formshown in FIG. 2B.

Monitoring of the battery voltage is accomplished by a battery monitornetwork 68 preferably formed of an operational amplifier 68A such asthat sold by National Semiconductor under model number LM10. Theamplifier 68A contains an accurate internal voltage reference (pin 1)and a general purpose amplifying stage (pins 2, 3 and 6). The outputreference (pin 1) is compared with the battery voltage applied at pin 3through a potentiometer 68R. The potentiometer 68R is adjusted formodulation cutoff upon the battery voltage dropping below 1.2 V. Whenthe battery voltage falls below this level (1.2 V) the output (pin 6) ofthe amplifying stage switches states to cut off the modulator 64. Thus,the electric field generator outputs a continuous unmodulated RF carriersignal (as shown at TP-1 in FIG. 2B). As will be discussed herein thesignal processor 26 is arranged to reject a continuous (unmodulated) RFcarrier signal and an indicator signal is unable to be generated. Inaddition, continuous transmission of the unmodulated RF carrier signalincreases the drain on the battery. This occurrence is advantageous inthat it minimizes the possibility of short term battery recovery bytemporarily turning off the generator.

As an alternative, a crowbar network may be used to monitor the batteryvoltage. As long as the battery supply remains greater than a referencevoltage applied to an operational amplifier in the crowbar network theoutput of the amplifier is not asserted. However, as soon as the batterysupply drops below the reference, the amplifier changes state to close aswitch which effectively shunts the battery to ground potential.

The rf voltage applied to the cover and conductive band relative to thebody of the operator generates an essentially isotropic electric field.This is due to the fact that the near field conditions ofelectromagnetism apply. The electric field lines terminate uniformly atthe surface of the conductive member disposed in the capacitive receptorantenna regardless of the orientation of the electric field generator.Consequently, the generation of nulls, voids or dead spaces in thesensitivity pattern of the capacitive receptor antenna 21 is avoided. Itmay be desirable to include in the generator a network to monitor theelectric field strength to insure that the field's strength remainsabove a predetermined level.

It should be noted that when several electric field generators areoperating simultaneously each transmits on a different crystalfrequency. Therefore, an arming indicator may be generated for eachgenerator used in the particular safety system. Of course, should thecircuitry which generates an arming indicator not be utilized in aparticular application the modulation of the field is superfluous. It isdesirable that the field be varied at a sufficiently low carrierfrequency such that the entire safety system may operate in the nearfield and that the capacitive coupling action between the fieldgenerator and the capacitive receptor antenna be almost entirely theresult of capacitive coupling between the electric field generator andthe capacitive receptor antenna.

Modifications to the electric field generator shown in FIG. 2A may benecessary to permit use of that circuit with the discrete logicimplementation of the signal processor. For example, it is necesssary toprovide a higher voltage source (e.g., nine volts) and to eliminate offperiod residual leakage current. Alternatively, the circuit shown inFIG. 2C may be used.

THE CAPACITIVE RECEPTOR ANTENNA

FIGS. 3A and 3B are isolated perspective views of two of the possibleconfigurations of a capacitive receptor antenna used in connection withthe operator safety system of the present invention.

The capacitive receptor antenna shown in FIG. 3A is adapted for use witha safety system arranged for the protection of an operator's hand whileusing a pipe flanging tool. The capacitive receptor antenna shown inFIG. 3B is adapted for use in connection with an elastomeric roll mill.Of course, the capacitive receptor antenna 21 may be arranged in anysuitable configuration compatible with the environment in which thesafety system 19 is used.

As seen in FIG. 3A the capacitive receptor antenna 21 is disposed withina substantially rectangular, nonconductive frame 70 that is convenientlymounted, as by pivots or hinges 72, adjacent to the face of a powertool. For example, the frame 70 is preferably pivotally mountable to thetool itself such that the machine tool forms part of the shieldingstructure. An aperture 74 is defined through which elongated objects maybe inserted into the jaws of the tool. No portion of the machine toolshould protrude through the aperture. The capacitive receptor antenna 21has a sensitivity which defines a hazard zone extending a predeterminedclose distance about the region forward of the frame 70. The boundary ofthe hazard zone is indicated by the dashed lines 78 in FIG. 3A. Entry ofthe portion of the person of the operator having the transmitter 20(FIG. 2) thereon into the hazard zone 78 causes the SAFETY LEVEL signal(on the line 30) to exceed the threshold and results in the generationof the first indicator signal. If desired, any predetermined number ofzones may be defined.

Further removed from the hazard zone is a similarly shaped arming zonedefined about the capacitive receptor antenna 21 by the dashed lines 80.Entry of the operator having the transmitter 20 thereon into the armingzone 80 results in the generation of the ARM signal on the line 36which, if it exceeds the threshold level, generates the secondindicator. The distances of the outer boundaries of the zones 78 and 80are controlled by the threshold settings in the signal processor. FIG.3C is a side elevational view taken along section lines 3C--3C in FIG.3A illustrating the structure of the capacitive receptor antenna 21. Thecapacitive receptor antenna 21 is formed of a conductive member 86electrically connected to a second conductive member 87. The members 86and 87 are formed of copper tape. A substantially L-shaped aluminumelectrostatic ground shield 88 is provided behind the conductive member86 plate. The shield 88 enhances sensitivity along the axis 90 of theantenna and minimizes the effects of electrical noise originating frompoints behind the shield 88. The members 86, 87 and the ground shield 88are disposed in a suitable nonconductive casing 70 such as vacuum-formed(extruded nonconductive) ABS plastic to provide mechanical protection.The self-test antenna 46 is an insulated wire loop supported in itsposition between the conductive members 86 and 87 and the electrostaticshield 88. The self-test antenna 46 is supported by a thermoplasticholder 94.

In the environment of a rubber rolling mill the capacitive receptorantenna 21 (FIG. 3B) takes the form of an elongated conductive tubularmember 84 mounted between end plates 85. The tubular member 84 extendssubstantially parallel to the axis of the rolls at a location just abovethe top and just past the roll farthest from the operator. In practiceit may be necessary to utilize compensating supplementary stub antennas84A and 84B disposed forwardly above the ends of the tubular member 84.The stub antennas are electrically connected to the tubular member.Again a hazard zone 78 is defined a predetermined distance forward ofthe capacitive receptor antenna and an arming zone 80 further removedtherefrom. It should be noted that although the details of the machinetool are not shown in FIG. 3B, the structure of the machine tool isneeded to produce the patterns as shown in that Figure.

SIGNAL PROCESSOR

The signal processor 23 comprises the receiver 24 and the control logic26. In the manner set forth herein the discrete logic implementation ofthe signal processor 23 is responsive to the magnitude of the inducedsignal to generate indicators when the induced signal magnitude exceedspredetermined thresholds. As exemplified herein, the indicators form thebasis of decisions aimed at the safe operation of the machine tool.

System operation is based upon the ability of the receiver 24 toselectively receive, identify (if the field is modulated) and amplitudedetect the induced signal over a wide dynamic range. The receiver 24 maybe a dual-conversion multi-channel type for providing video level outputsignals which are nonlinear functions (typically logarithmic) of theradio frequency input signal level. The receiver is operative togenerate a signal (SAFETY LEVEL) that exhibits a first functionalrelationship to the distance between the electric field generator 20 andthe capacitive receptor antenna 21 and a second signal (ARM) whichexhibits a second functional relationship with the distance between theelectric field generator 20 and the capacitive receptor antenna 21. Theconvert-up and convert-down configuration of the receiver 24 (which isdescribed in connection with FIG. 4A) not only facilitates settingdifferent frequencies into the receiver and provides high imagefrequency rejection, but it also enables the receiver to respond withhigher sensitivity to the weak signals for establishing arming distancewhile simultaneously responding with greater sensitivity to the closerange signals for enhanced reliability in handling the signal levelswithin the hazard zone. Tuning the receiver to a predetermined frequencyis simply accomplished by changing the first local oscillator crystalfrequency and realigning the corresponding RF radio frequency filter.

Referring to FIG. 4A, shown is a block diagram of the generalconfiguration of a single channel of the receiver 24 used in oneembodiment of the invention. The front end comprises a tuned preselector106 which impedance matches the capacitive receptor antenna 21 via atransmission line 22 to all installed receiver channels, such as the twoadditional channels depicted by leads 108A and 108B, wherein the numberof channels installed matches the number of electric field generators 20in use. Filter 110 provides a bandpass of around one hundred kHz upon a1.3 MHz center frequency and passes the received signal, afteramplification by a first amplifier 112, to a first mixer 114, typicallya MCLSCA-1 sold by MiniCircuits, a division of Scientific ComponentsCorporation, where it is mixed with a twelve MHz signal from aselectable, crystal-controlled oscillator 116. The mixer 114 is "highside" injected with the signal from the oscillator 116 which has afrequency selected to equal the sum of the 10.7 MHz first IF frequencywith the RF antenna frequency generated by the particular electric fieldgenerator 20, in this case, 1.3 MHz.

The output of mixer 114 is further amplified by a second amplifier 120and passes through bandpass-filter 122 having a characteristic passbandof eight kHz and centered at 10.7 MHz. The SAFETY LEVEL and ARM signalsare generated by superimposing a predetermined number of currentsrepresenting the induced signal from the capacitive receptor antenna 21.A first video detector amplifier 124, connected to the output of filter122, generates a first detected signal current through resistors 126 and128 for input to ARM and SAFETY LEVEL buffers 130, 132, respectively.

The second detected signal current is derived in the following manner:An amplifier 134 amplifies the output signal from the filter 122 andpasses it to a 10.7 MHz IF filter 136 having a passband of eight kHz foradditional receiver selectivity bandpass narrowing. An amplifier 138further amplifies the output signal of the filter 136 and applies it toone input terminal of a second mixer circuit 140 similar to the mixer114 while a crystal oscillator 142 generates an 11.125 MHz signal forinput to the other input terminal of the mixer 140. The resultant mixeroutput signal is filtered by a 455 kHz IF filter 144 (having a passbandof fifteen to twenty-five kHz) before it is detected by a second videodetector amplifier 146 which in turn generates a second detected signalcurrent through resistors 148 and 150, respectively.

In a similar fashion, the third and fourth detected signal currents arederived from amplifier-filter-video detector loops 154, 156 and 158 and160, 162, 164, respectively to generate detected signal currents throughrespective resistors 166, 168 and 170, 172. Each of the filters 156 and162 have a passband of fifteen to twenty-five kHz and a center frequencyof 455 kHz. The values of the resistors 128, 150, 168 and 172 areselected to cause the first electrical signal (SAFETY LEVEL) to followthe lower curve in FIG. 4B. The values of resistors 126, 148, 166, 170are selected to tailor the second signal (ARM) to follow the upper curvein FIG. 4B. The resistors have values which range typically from 50Kohms to 1 M ohms. By way of example, the straight line in FIG. 4Brepresents the logarithmic condition where all resistors have identicalvalues, in this case 820K ohms. It also becomes evident that theresponse curves can assume any shape to cover a specified machine safetyrequirement. In the instant example, the ARM signal output from thebuffer 130 has the steepest characteristic response when signal strengthis weakest. This would correspond to the induced signal from thecapacitive receptor antenna 21 when the operator is just entering thearming zone 80 (FIGS. 3A and 3B). On the other hand, the SAFETY LEVELcurve has the steepest slope when the electric field generator 20 isclosest to the capacitive receptor antenna 22. This would occur when theoperator is nearest to the hazard zone 78 (FIGS. 3A and 3B). Since thereceiver 24 is made to be position-sensitive to the high leveltransmitter signals close to the protected machine the limit of thehazard zone 78 remains essentially fixed in space. Deviations from thepredetermined performance standards are detected by means of theself-test system discussed herein.

The control logic 26 functions to generate the MACHINE ENABLE andMACHINE DISABLE indicators. These indicators are based on the magnitudeof the amplitudes of the SAFETY LEVEL and ARM signals, and theidentification of the modulation characteristics of one of thesesignals. In addition the results of the self-test system may also beused to form these signals. These signals are used to assert overridingcontrol over the machine operator's freedom to energize the machine orto work in the vicinity of its hazard zone.

FIG. 5 shows in schematic diagram form the basic features of the controllogic circuitry comprising both digital and analog components used inthe embodiment of the invention in which the signal processor isimplemented in discrete logic form. The ARM and SAFETY LEVEL square wavesignals shown on the lines 36 and 30, respectively, are typical ofsignals formed by the receiver 24. These pulse signals, which haveamplitudes that vary above a zero-volt base line, are functions ofsignal strength and are input to sample-and-hold circuits 180 and 182,respectively.

A zero-crossing detector 184 monitors the ARM signal waveform and thenapplies its squared-up output signal to the control terminals of bothdetectors 180 and 182 in order to synchronize their sample-and-holdfunctions with the occurrence of each pulse in the ARM signal waveform.The output signal of zero-crossing detector 184 is a constant amplitudepulse train wherein each pulse has a duration identical to that of thecorresponding monitored ARM signal pulse. Suitable for use as thezero-crossing detector is a device manufactured by Motorola under modelnumber MC3403.

In addition to the control terminals of sample-and-hold detectors 180and 182, the output of zero-crossing detector 184 is connected to signalidentification network 186 comprising a modulation characteristic(recurrence rate and duty cycle) discriminator 188 and percentageacceptance integrator 190. The discriminator 188 detects the presence ofa signal having a duty cycle and recurrence rate identical to that ofthe electric field generator. The percentage acceptance integrator 190determines whether the modulation characteristic of the detected signalmeets the predetermined reference modulation characteristic and whetheror not the signal is transient. A detailed circuit description of thesignal identification network 186 is given in FIG. 6A.

The sampled and held signal levels at the output of detector 182 areapplied to one input terminal of a SAFETY threshold detector 194 with alevel set 196 attached to the other. The detector 194 is typically acomparator or an operational amplifier such as that sold by MotorolaInc. under model 3403. The level set 196 provides a constant referencevoltage level that is no higher than that necessary to disable themachine at a point no closer to the machine than the outer limit of thehazard zone, FIGS. 3A and 3B. If the sampled signal from the detector182 exceeds the reference at the inverting input of the comparator 194,the indicator MACHINE DISABLE signal on the line 32 is generated (whichis independent of the ARM signal identification function performed bythe network 186). The indicator MACHINE DISABLE signal is applied on aline 198 to the self-test network 46 (FIG. 7A). In a similar fashion,the ARM threshold detector circuit 202 having a level set 204 producesan indicator precursor signal to one terminal of NAND gate 206 when thetransmitter signal is sufficiently strong at the outer boundary of thearming zone, FIG. 3. The device 202 is similar to the device 194.

Provided the induced signal has been acceptably identified, a signalfrom the output of signal identification network 186 will be applied tothe second terminal of the NAND gate 206 resulting in a logic low signalARM DETECTED at the output terminal of the gate 206. The ARM DETECTEDsignal on the line 208 is applied to the self-test network 46 (FIG. 7A).

The ARM DETECTED signal in turn results in an output on the line 38 ofthe indicator MACHINE ENABLE to the machine interface controller 28,unless prevented by one of the gates 212, 214 or 216. AND gate 212functions to prevent machine enabling while a self-test is in progress;AND gate 214 functions to prevent machine enabling should self-testingshow a fault in any channel of the machine-guard system; and AND gate216 prevents machine enabling before a self-test has been completed uponstart-up of the machine. If the system fails a self-test sequence or ifa self-test is required, a NOR gate 218 forms a RESET REQUIRED signalwhich indicates to the operator that he must reset the controllersbefore machine operation can again be allowed.

FIG. 6A shows the circuit details for the signal identification network186 with explanatory waveform characteristics of the signal conditionsat the corresponding numbered locations being shown in FIG. 6B.

This circuit operates to create a repetitive window, or gate, timed insuch a way to capture one of the two edges defining a repetitive pulsesignal that characterizes the induced signal having the predeterminedmodulation characteristic. During the gating interval, the leading ortrailing edge (but not both) of each pulse in the induced signal will begated, but pulses not having edges falling within this period will not.In addition to modulation characteristic discriminator circuit 188, thenetwork includes a percentage acceptance integrator circuit 190 fordetermining whether the number of pulse edges detected within aprescribed time limit has met the acceptance criterion.

The input signal, the waveform at TP-1 (FIG. 6B) from zero-crossingdetector circuit 184, is input to a comparator 230, typically configuredfrom a device sold by Motorola under model number MC13403CP operationalamplifier, where it is compared with a first reference level, typicallyseven volts. The comparator 230 inverts the signal and passes it to afrequency determining one-shot circuit 232 through a pulse-shapingnetwork 234, comprising the combination of a 0.01 microfarad capacitor234C, a 10K ohm resistor 234R and a diode rectifier 234D, typically aIN4148.

In operation, the pulse-shaping network 234 forms a negativeedge-trigger pulse from the leading edge of each input pulse and appliesit to the inverting terminal of the amplifier 232, typically aMC13403CP, across a 56K ohm resistor 236. A second reference level,typically nine volts, biases the input signals to both the amplifier 232and an operational amplifier 240 (identical to the amplifier 232) sothat two volt pulse signals, each having constant duration, are formedin response to the trigger pulses as depicted by the waveform at TP-2(FIG. 6B). The pulse duration is a function of both the value of acapacitor 242C and the resistance value of the duty cycle setpotentiometer network 242R, comprising 100K ohm and 10K ohm resistors,respectively. Pulse duration is adjusted manually such that it is justshort of the ideal transmitted pulse length.

In a similar fashion, the pulse-shaping network 244, comprising a 0.001microfarad capacitor 244C, a 1M ohm resistor 244R and a diode rectifier244D (identical to the rectifier 234D), generates a second negative edgetrigger signal upon the occurrences of the trailing edges of each pulsein the waveform at TP-2 and impresses these trigger pulses onto theinverting terminal of the amplifier 240 across 56K ohm resistor 246. Theconstant value duration of the generated pulses by the window-one-shotcircuit 240 is accordingly established by the resistance values of theduty cycle tolerance and recurrence rate tolerance set potentiometer245R, comprising 100K ohm and 47K ohm resistances, respectively, plusthe value of capacitor 245C, typically 0.01 microfarad. The windowpulses so generated, waveform at TP-3 (FIG. 6B) are arranged to overlapthe trailing edge of the hypothetical ideal transmitted pulses asdepicted above the first waveform by the alphanumerics T1, T2, T3, T4,T5, T6 and T7.

Flip-flop 250, typically a device sold by Motorola under model numberMC14013CP, serves to sample and hold the trailing edge occurrences ofinput waveform at TP-3 applied to its data terminal wherever theseoccurrences are clocked by the trailing edges of waveform at TP-1. It isevident that the duration of the positive portions of the square-waveoutput waveform at TP-4 at terminal Q of flip-flop 250 is indicative ofthe degree of correlation between the received signal and thetransmitter signal.

The percentage acceptance integrator circuit 190 comprises an integratorcircuit 252 and a serially connected threshold circuit 254. The functionof the percentage acceptance circuit 190 is to measure the degree ofcorrelation between the actual signal and the ideal transmitter signalas indicated by waveform at TP-4 and generates a logic value signal whena predetermined degree of correlation has been attained. This lattersignal, the waveform at TP-6, controls the NAND gate 206 (FIG. 5) andthus the passage of the indicator MACHINE ENABLE signal to the machineinterface circuit 28. The integrator 252 uses an operational amplifier,typically a device sold by Motorola under model number MC13458CP, and anRC network consisting of a 120K ohm resistor 252R and 0.1 microfaradcapacitor 252C connected to its noninverting terinal to integrate thelogic level correlation signal, at waveform TP-4, and generate a degreeof correlation signal waveform at TP-5. The waveform at TP-5 istransmitted through a 33K ohm resistor 256 to the threshold circuit 254which applies a voltage reference, provided by the voltage divider 254R,formed by the 120K ohm and 470K ohm resistances, respectively, for acomparison value to the inverting terminal of an operational amplifier254, such as a device sold by Motorola as model number MC13458CP. In theexample shown, the integrator output signal at TP-5 is not sufficient tocross the approximately eighty percent acceptance level thresholdestablished by resistors forming the divider 254R and consequently thewaveform at TP-6 does not go positive.

To summarize, the function of the signal identification network is toisolate a signal of interest from an electromagnetic interference noisebackground or from other signal sources, in order to establish thevalidity of a received signal.

SELF-TEST NETWORK

The self-test network 42 (FIG. 1) is operative to apply a signal ofknown intensity to the self-test antenna 46 which varies at thepredetermined radio frequency and exhibits the predetermined modulationcharacteristic to verify the operation of the machine tool safetysystem. FIG. 7A is a schematic diagram of the self-test network whileFIG. 7B is a timing diagram illustrating the chain of events that occurduring a self-test sequence.

The self-test sequence occurs during system start-up or commences with aloss of the ARM DETECTED signal on the line 208 (FIG. 5). The signal ARMDETECTED on the line 208 is normally low when the ARM signal isasserted. A loss of the ARM DETECTED signal would result, e.g., when theoperator temporarily opens the switch 56 (FIG. 2A) on the electric fieldgenerator 20, when the equipment is turned-on or when a signal from theelectric field generator is either not present or is out of range. Thisoccurrence causes the signal output from the inverter 260 (ARMDETECTED-NOT) to change state to a logic low condition and initiate athree-stage timing sequence.

When the output of the inverter 260 changes state to a logic low, thistransition couples a negative-going edge through a capacitor 261C to theinverting terminal of an operational amplifier 264, such as a devicemanufactured by Motorola and sold under model number MC13403 CPconfigured as a one-shot. The output from the one-shot 264 provides thefirst stage timing signal which establishes a predetermined timeinterval, conveniently seventeen seconds, between self-test sequences.The slope of the negative-going edge of the signal input to theamplifier 264 is governed by the values of the capacitor 261C and aresistor 261R. Since the reference voltage V₂ to the amplifier 264 isnormally higher than the reference voltage V₁, the output of theamplifer 264 is normally in a logic low state. However, the appearanceof a negative edge at the inverting terminal of the amplifier 264changes the output thereof to a logic high state. The duration of thelogic high output is maintained by the values of a capacitor 262C and aresistor 262R. Since the voltage level corresponding to a logic highstate at the output of the amplifier 264 is coupled to its noninvertinginput through the capacitor 262C, the voltage at that terminal remainshigher than the reference voltage V₂ for a period determined by the rateof discharge of the capacitor 262C through the resistor 262R. The diode261D serves to terminate the logic high state at the output of theamplifier 264 whenever the output of the inverter 260 goes high(indicating that the ARM signal has been detected), regardless ofwhether or not the one-shot 264 has timed out. The diode 262D thenserves to rapidly discharge the capacitor 262C to enable the amplifier264 to be ready to generate a logic high output state upon the next lossof the ARM signal. Should the signal from the transmitter be regainedafter initiating the self-test sequence, the diode 261D serves to blockthe initiation of a new self-test timing sequence.

When the pulse from the one-shot 264 times out, the negative-goingtrailing edge thereof triggers a one-shot 266 similar to the device 264.The one-shot 266 outputs a second timing signal, a pulse (logic highstate) having a second predetermined duration (preferably 0.8 second)which is applied to various points in the self-test network.

Briefly described, a capacitor 266C conducts the negative edge of thefirst timing signal from the operational amplifier 264 to the invertingterminal of the operational amplifier 266 by way of a resistor 266R andcauses the voltage at the inverting input of the amplifier 266 to dropbelow the voltage at its noninverting terminal. Thus a logic high stateis present at the output of the one-shot 266. The length of time thislogic high state is sustained is determined by the values of a capacitor267C and a resistor 267R, similar to the functioning of the capacitor262C and the resistor 262R for the operational amplifier 264. Resistors266R and 263R and the capacitor 266C also determine the slope of thenegative-going edge of the input triggering signal from the amplifier264. A diode 267D provides a discharge path for the capacitor 267C torapidly prepare the amplifier 266 for the next timing cycle. A diode269D serves to admit a logic high signal from an AND gate 280 to abortthe timing of the amplifier 266 and the initiation of the self-testsequence through the one-shot 264, mentioned earlier to meet normaloperating demands.

The output from the one-shot 266 triggers a test oscillator 268 througha power amplifier 270, such as a device manufactured by Motorola undermodel number 2N2222. The test oscillator 268 is configured similar tothe electric field generator 20 (FIG. 2) and generates excitationsignals which, when applied to the self-test antenna 46 through anattenuator 271, stimulates the receiver to the same response as wouldactual transmissions from the electric field generator 20. Accordingly,upon the occurrence of the pulse from the one-shot 266, the MACHINEDISABLE and ARM DETECTED signals should appear on the lines 198 and 208(FIG. 5).

The third stage timing signal is provided by an operational amplifier272 and is delayed from the self-test pulse generated by the one-shot266 for a time sufficient to cover the system propagation delay.Operational amplifier 272 acts as a delay pulse generator and isactivated by the leading positive-going edge of the second timing signalgenerated by the one-shot 266. A network comprising a resistor 272R anda capacitor 272C determines the delay time. This third timing signal(the delayed pulse) is applied through an AND gate 292 and provides aclock input edge to two flip-flops 296 and 298. Suitable for use as theflip-flops are devices manufactured by Motorola under model number MC14013CP. The flip-flop 298 is set with its Q output to a logic low statethrough a capacitor 298C and a resistor 298R at turn-on of the system.The other flip-flop 296 is used for pass/fail of the self-test sequence.If the signal at the data input of the flip-flop 296 is high and thelater-arriving positive-going clock edge appears, self-test is passed.If the data input is low and the later-arriving clock edge appears,self-test fails. When logic low states are present at either one or bothQ output terminals of the flip-flops 296 and 298, the AND gates 214 and216 (FIG. 5) operate to disable machine operation until cleared by amanual reset when normal system operation is resumed. An AND gate 300prevents the transfer of data pulses during normal operation whereas anAND gate 302 prevents the transfer of clock pulses during normaloperation.

The signals from the one-shot 266 and from the delay pulse generator 272are joined by a NOR gate 304. The output of the gate 304 is applied tothe control AND gate 280 and to a NAND gate 306. When activated, theoutput from the gate 306 prevents the machine tool from becomingoperable during the self-test sequence by generating a control signalthrough the gate 302 which disables the gate 292 to prevent clocking ofthe flip-flops 296 and 298.

The AND gate 292 combines the output from the delay pulse generator 272with the output from the AND gate 302. The output of the gate 292generates a pulse, the leading edge of which simultaneously clocks theflip-flops 296 and 298.

The data inputs to the flip-flops 296 and 298 are derived from theoutput of the AND gate 300 which itself derives its inputs from theone-shot 266 and the output of an AND gate 308. The output of the gate308 is asserted if the simulated operating signal from the testoscillator 268 results in the generation of both an ARM DETECTED andMACHINE DISABLE signal.

The AND gate 302 provides a logic high level output when self-teststandard has been met by the signal processing sections, FIGS. 5 and 6A.When the AND gate 300 receives a logic high signal from the gate 308 anda logic high signal from the one-shot 266 a logic high state is producedat the output of the gate 300 indicating a test is in progress and thatthe test conditions have been met. The logic high output from the ANDgate 300 applies to the data input terminal of the flip-flops 296 and298 a logic high which is immediately clocked by the later-developedclock pulse rising edge emanating from the gate 292. The gate 292outputs a logic high when the delay timer 272 pulse is high and the ANDgate 302 output is high. The gates 302 and 306 together serve to blockclock pulses if a self-test is not in progress. In such a case, the gate302 outputs a logic low to prevent the transfer of clock pulses from thetimer 272 to the clock input terminals of the flip-flops 296 and 298.With the exception of those instances when the output of the inverter260 is high, the timer 264 output is low, and the delay timer 272 outputis high, all other combinations of the various timer and arm functionlevels are rejected.

The NOR gate 304 serves to disable the AND gate 280 during self-test,since the timers 266 and 272 together or independently are in a logichigh state during this period. Consequently, the signal output from thegate 280 goes to a logic low to indicate SELF TEST IN PROGRESS-NOT onthe line 278 to the gate 212 (FIG. 5).

A logic high signal at the Q output at the flip-flops 296 and 298 isapplied over output lines 314 and 316 to enable gates 214 and 216 (FIG.5). Thus, if the system passes each of the self-tests, the indicatorMACHINE ENABLE on the line 38 may be asserted and the machine tooloperated. The Q-NOT terminals of the flip-flops 296 and 298 are appliedover lines 318 and 320, respectively, to the gate 218 (FIG. 5)indicating a reset of the system is required (RESET REQUIRED). Further,failure of a self-test disables the gates 214 and 216 thus preventingthe assertion of a MACHINE ENABLE signal. It should be noted that whenthe system is initially energized a RESET REQUIRED appears.

If the particular embodiment of the machine tool safety system utilizestwo channels, the self-test networks for those channels will interact byapplying the output of the one-shot 266 to the corresponding gate 306 inthe self-test network for the second channel.

MICROCOMPUTER CONTROLLED RECEIVER AND SIGNAL PROCESSOR

Shown in FIGS. 8 through 17 is a firmware-based microcomputer-controlledimplementation of the signal processor arrangement useful in connectionwith the present invention. The microcomputer-controlled receiver 24'used in connection with the control logic network 26' contains an arrayof amplifier stages and peak detectors which are sampled under thecontrol of the microcomputer. In this manner themicrocomputer-controlled signal processor 23' measures the magnitude ofthe sampled signal peaks at each amplification stage and selectivelyconverts the results into decibel form for comparison with predeterminedand calibrated threshold levels to generate machine control indicators.

With reference to FIG. 8 a generalized block diagram of themicrocomputer-controlled signal processor for a two channel safetysystem is shown. In FIG. 8 the signal from each of the electric fieldgenerators worn by the machine operator is carried from the capacitivereceptor antenna (FIG. 3) over the coaxial cable link 22 into the highimpedence input of a common radio frequency amplifier 330. The amplifier330 is configured to provide approximately ten decibels of gain. Theoutput of the amplifier 330 is carried over lines 332 each leading to areceiver 24', with one receiver 24' being provided for each of the wristtransmitters utilized in the particular implementation of the invention.

Although shown in FIG. 8 are two receiver channels 24'A and 24'B, it isto be understood that any predetermined number of receivers,corresponding to the number of electric field generators utilized in theparticular implementation of the machine tool safety system aredeployed. In the description that follows herein only the receiver 24'Aand associated elements for one channel of operation (corresponding toone electric field generator) are described. However, the circuitconfiguration and operation for other receiver channels (if provided)are identical. (In the program listing appended hereto and made parthereof, the two channels A and B are referred to as channels 0 and 1,respectively. In the discussion of the program which follows, the suffix"X" following a register label indicates that the register is associatedwith the channel of interest.)

Each receiver 24' is operatively associated with the microcomputer 460(FIG. 12) disposed within the control logic network 26' by control lines334 and 336. In addition, each receiver 24' is operatively connected toa multiplexing analog-to-digital converter 338 by lines 340, 342, 344and 346. Suitable for use as the converter 338 is a device sold byNational Semiconductor Corporation under model number ADC0809CCN. Theconverter 338 is connected to the microcomputer for its synchronizationand timing over a set of control lines which are collectively indicatedby the reference characters 348 and 349. In addition, the eight-bit datafrom the converter 338 flows unidirectionally over a bus 350 to themicrocomputer 460, when addressed over lines 470, and transmitting aconvert complete signal on a line 461. The converter 338 is alsoconnected to an array of potentiometers 352 (ARM), 354 (SAFETY LEVEL),356 (SELF-TEST HIGH) and 358 (SELF-TEST LOW). The microcomputer 460(FIG. 12) is also connected to a self-test generator 42' over controllines 362 and gain control lines 364. The signal from the self-testgenerator 42' is carried over a transmission line 44 to the self-testantenna 46.

The control logic network 26' receives machine tool status signals andservice requests over an array of conductors 366. Control indicators,such as MACHINE ENABLE and MACHINE OVERRIDE are output from the controllogic network 26' to the machine interface 28 over an array of outputlines 368.

Each receiver 24' includes a first mixing stage 372 and a second mixingstage 374 coupled through a filter network 376. The output of the secondmixing stage 374 is serially applied through an array of amplifiers 378,380 and 382, respectively. The output of the second mixer 374 and eachamplifier stage 378, 380 and 382 is connected to a peak detector 384,386, 388 and 390, respectively. The peak detectors are respectivelycoupled to the converter 338 over the lines 340, 342, 344 and 346.

FIG. 9 is a detailed schematic diagram of the first and second mixingstages 372 and 374, respectively and of the filter network 376. Each ofthe mixing stages includes a mixer element 396 such as that manufacturedby National Semiconductor and sold under model number LM-1496N. Acrystal controlled local oscillator 398 is connected by a line 400 tothe mixing element in each of the stages. The stages 372 and 374 operatein sequence to translate to a first intermediate frequency of 10.7megahertz down to one hundred kilohertz. Both stages use high sideinjection. The filter network 376 is a 10.7 megahertz crystal filterprimarily used to reject the adjacent channel. The output of the secondstage mixer 374 is coupled by a tuned transformer 408 to the firstamplifier stage 378 (FIG. 10).

As shown in FIG. 10 each amplifying stage 378, 380 and 382 is anintermediate frequency amplifier designed to pass the one hundredkilohertz signal output from the second mixer stage 374. Suitable foruse within each amplifying stage is a current mode saturating typeoperational amplifer such as that manufactured by RCA under model numberCA3080. This operational amplifier is selected because it providesexcellent linearity and (fast) recovery characteristics well into itssaturation limit. Each IF amplification stage is configured to give 19.5decibels of gain. The last operational amplifier (in the amplificationstage 382) will saturate first as signal strength progressivelyincreases and each of the preceding amplifier circuits saturatesprogressively toward the front end of the receiver.

FIG. 10 also shows a detailed schematic diagram of the peak detectorcircuit 384. Each of the other peak detector sample and hold circuits386, 388 and 390 are identical to the detector 384. Peak detector 384includes a peak-detect sample and hold network generally indicated byreference character 412 and a comparator arrangement generally indicatedby reference character 428.

The sample and hold network 412 includes a comparator 414 such as thatmanufactured by National Semiconductor under model number LM339. Theinput of the comparator 414 is connected to the output terminal of thecoupling transformer 408 or the preceding IF stage through a currentlimiting resistor 418. The output of the comparator 414 is connectedthrough a diode 420 and a resistor 422 to a capacitor 424.

The comparator network 428 includes first and second comparators 430 and432, respectively. The noninverting inputs of each of the comparators430 and 432 are connected to a positive biasing voltage. The invertingterminal of the amplifier 430 is connected over the control line 334(DUMP) to the microcomputer (FIG. 12). The inverting terminal of thecomparator 432 is connected over the control line 336 ENABLE output fromthe microcomputer. The output of the comparator 430 is connected to thecapacitor 424 and over the line 340 to the analog-to-digital converter338. Similarly, the output signals from the peak detectors 386, 388 and390, respectively pass over lines 342, 344 and 346 to the converter 338.

As is discussed herein in connection with the operation of themicrocomputer controlled receiver and control logic network, before avoltage to be measured is impressed across the capacitor 424 themicrocomputer generates a ten microsecond DUMP signal applied on theline 334 to allow the charge resident on a capacitor 424 to drain toground via the comparator 430, as indicated by the arrow 434. Thereafterthe microcomputer 460 disables the comparator 432 by signal over theline 336 thus enabling the peak detector comparator 414. Morespecifically the change in the output state of the comparator 432 allowsthe comparator 414 to function normally as a peak detector. So long asthe positive input of the comparator 414 is greater than its invertingterminal input, the comparator 414 is in an open collector stateallowing current to flow through the resistor 419, the diode 420 and theresistor 422 to charge the capacitor 424. Thus, at the end of a sampletime the voltage across the capacitor 424 represents the positive peakvalue of the signal at the positive input of the comparator 414.

With reference to FIG. 11 the self-test signal generator 42' useful in atwo channel, microcomputer controlled, signal processor is shown. Theself-test signal generator includes two crystal controlled oscillators438 and 440, respectively, each associated with one receiver channel.The oscillators drive clocked flip-flops 442 and 444, respectively. Theflip-flops 442 and 444 are each configured in a divide-by-two mode.Suitable for use as flip-flops 442 and 444 are CD devices manufacturedby RCA and sold under model number 4013. The reset terminals of each ofthe flip-flops is controlled by a control line 362 from themicrocomputer 460. Pulsating outputs from the flip-flops 442 and 444(which have a frequency of one-half the rate of the oscillators 438 and440) are resistively added to feed a common output driver network 448.The driver network 448 typically consists of a 2N2907A transistoramplifier 449. The output of the driver network 448 is applied through awide band transformer 450 to a gain control network 452 configured froma resistor-diode bridge. The control signal from the microcomputer onthe gain control line 364 is applied to the base of a switch 454,typically a 2N2222A transistor. The switch 454 serves to select theparticular high level (upper path) or low level (lower path) self-testoutput signal signalled by the microcomputer. Potentiometers 356 and 358(FIG. 8) connected to the analog-to-digital converter 338 are adjustedto match within plus or minus one dB the self-test high and low levelsignals. Once set, deviations of the self-test signal strength more thanplus or minus two dB will be recorded as a self-test failure. Theself-test signal from the self-test signal generator is applied to theself-test antenna 46 through the coupling transformer 456. Theappropriate square wave oscillator 442 or 444 for the channel to betested is controlled by the control lines 362A and 362B from themicrocomputer 460. Of course, depending upon the number of channels usedin a particular implementation of the safety system, a correspondingnumber of self-test signal generator channels may be provided.

With reference to FIG. 12, a block diagram of themicrocomputer-controlled control logic network 26' is shown. The controllogic network 26' includes a microcomputer 460, such as a microcomputerchip manufactured by Intel Corporation and sold under model number 8039.Timing for the microcomputer 460 is derived from a suitable crystal 462,typically with a resonant frequency of six megaHertz. The microcomputer460 communicates over an eight bit data bus 464 with an address latch466 and with a program memory 468. Program addresses from the latch 466are applied to the memory 468 over an eight bit address bus 470.

The output bus 350 from each analog-to-digital converter 338 used in thesystem is connected to the address bus 464 of the microcomputer 460.Control signals to each of the converters 338 on the conductors 348 arederived from a converter control network 472 which includes a readcontroller logic 474 and a divide-by-two circuit 476 to provide theconverter 338 clock signal. Inputs to the read controller logic 474 arederived from one address bit carried from the address latch 466 on aline 478 and from the microcomputer 460 on a line 480. The dividercircuit 476 is driven by a clock signal on a line 482 from themicrocomputer 460. Converters 338A and 338B (in a two-channel system)are read one at a time by a signal from the read controller logic 474over the control lines 348A or 348B, respectively. A CONVERT COMPLETEsignal is sent to the microcomputer 460 by one (and thus the other) ofthe converters 338 on the line 461.

Information regarding the status of the machine tool is applied to themicrocomputer 460 over the conductors 366 input at port Pl. Controlsignals to the receiver on the lines 334 and 336 (FIGS. 8 and 10)emanate from port P2 of the microcomputer controller 460. Signals to themachine interface 28 (FIG. 8) from the microcomputer 460 are carried bya bus 488 to an input/output expander 490, such as that sold by Intelunder model number 8243. The expander 490 communicates with a driver492, as a device sold by Sprague under model number ULN 2803, which inturn communicates with the machine interface over the conductors 368.Outputs from the control logic 26' include the MACHINE ENABLE andMACHINE OVERRIDE indicators. Also output from the expander 490 are thecontrol lines 362 and 364 to the self-test network 42' (FIG. 11).

The operating mode for the machine tool safety system is selected by anoperator through the mode select switch 494. The four availableoperating modes include: processing of signals from transmitter channelA only; processing of signals from transmitter channel B only;processing of signals from transmitter channels A or B (e.g., valid whencontrolling an elastomer mill); and processing of signals fromtransmitter channels A and B (e.g., valid when controlling a pipeflanging machine). The microcomputer 460 may be reset, as when enteringa mode change or restarting the program after a self-test failure,through a reset line 495.

The operation of the microcomputer-controlled receiver and control logicshown in FIGS. 8 through 12 may be explained in connection with thetiming and register sequences diagrams shown in FIGS. 13A through 13Eand with the flow charts shown in FIGS. 14 through 18.

The signal output from the electric field generator is basically aninterrupted continuous carrier wave (FIGS. 2B and 13A) modulated inaccordance with a predetermined modulation characteristic to exhibit apredetermined recurrence rate and duty cycle. In practice, a modulationcharacteristic using a seventy Hertz recurrence rate and fifty percentduty cycle is preferred. This signal for each channel is linearlyprocessed, filtered and detected by the receiver 24' using theconfiguration of elements shown in FIGS. 9 and 10 and operating underthe control of the microcomputer 460. After conditioning the inducedsignal detected by a given channel typically appears as shown in FIG.13B. The processing of the information contained in the signal receivedby the receiver 24' is performed by the signal processor 23' operatingunder the control of the microcomputer 460. The essential steps insignal processing include: (1) synchronizing the processor with thetransmitted signal; (2) determining whether a "lock" exists, that is,whether the received signal exhibits the appropriate modulationcharacteristic; (3) signal level averaging to determine the amplitude ofthe received signal; and (4) determining the existence of ARM and SAFETYLEVEL conditions.

As a general overview of these processes, reference is invited to FIGS.13B through 13D. The detected signal for a given channel (FIG. 13B) issampled during predetermined sample intervals (FIG. 13C). If themodulation characteristic utilizes the preferred recurrence rate ofseventy Hertz (period of fourteen milliseconds) there are on the averageseven, two-millisecond sample intervals during each fourteen millisecondperiod (FIGS. 13B and 13C). During each sample interval the detectedsignal is logarithmically averaged and quantized by the processor byassigning a digital bit (usually a binary 1) if the amplitude of thesampled signal is greater than a running average. The historical resultsof the quantization for the detected signal during each sample intervalare stored in a first register (DATAX register). A second register, theSYNCX register, is also maintained for each channel. Synchronizationoccurs (and an indication thereof is stored in the SYNCX register) onlyif a predetermined pattern of bits is developed and stored in the DATAXregister. A "lock" may be generated only if the appropriate bit patternis stored in the DATAX register at the end of the last sample intervaland if synchronization has occurred after a minimum of twenty-four pulserepetition intervals. Once "lock" is achieved, the average peakintensity is calculated. This average is maintained in a suitable PKAVGXregister and is used to determine whether the ARM condition has beenmet. Once the ARM condition is met, the SAFETY LEVEL condition isflagged should two successive signal samples exceed the preset SAFETYLEVEL threshold. A complete listing (in assembly language for the IntelCorporation 8039 microcomputer) of the program used by the microcomputerto effect the flow diagram is attached to and made part of thisapplication.

The sequence of activities occurring during each sample interval isshown in FIG. 13D. The sample intervals are asynchronous to the incomingsignal to which synchronization and quantization is required. The twomillisecond sample interval is subdivided into two subintervals of 0.24milliseconds and 1.76 milliseconds, respectively. During the firstsubinterval (FIG. 14) each of the detectors 384, 386, 388 and 390 (FIGS.8 and 10) are cleared by asserting a DETECTOR DUMP signal on the line334 (FIG. 10) to drain the charge on the capacitor 424 disposed in eachpeak detector stage. Thereafter, the detector circuitry is enabled byasserting an ENABLE signal on the lines 336 to the detectors (FIG. 10).(References to the Appendix are indicated in the flow diagrams by theterm "APP. PG. ").

As seen from FIG. 13D, during the second subinterval the samplemeasurements as affected by the detectors are disabled by terminatingthe ENABLE signal on the lines 336. Thereafter, the analog-to-digitalconversion of the output of each detector is requested by themicrocomputer 460, starting from highest order detector (the detector390) and proceeding toward the front of the receiver 24'. The convertedvalue is sent to the microcomputer 460 on the bus 350 and stored in asuitable register, thus quantizing the received signal to a resolutionof plus or minus 0.5 dB. The output of the highest-ordered detector inthe chain that is not saturated is retained and converted to alogarithmic value lying within the segment of the receiver's zero toeighty-two dB response range allocated to that detector.

Once the detectors are polled, the running average of the receivedsignal in decibels is computed and maintained as an exponentialintegration of successive samples to minimize storage requirements andprocessing time using the routine CMPAVG (FIG. 15). The running averageis maintained in the upper two bytes of a three-byte storage registerSIGAVX. The lower byte contains the two's-complement of the old runningaverage divided by a number 2^(N), where N typically equals five. Aftereach new sample is received and converted into decibel form, a newaverage value is calculated. The new average is the sum of the newsample value plus the product of the old average and a number(1-2^(-N)). This technique provides integration on an exponential curvesimilar to that obtained by integrating voltage in a resistor-capacitorelectrical network. The equivalent time constant is determined by aspecified value N contained in a data register GENCNT. When the CMPAVGsubroutine (FIG. 15) is called, the time constant is the product of thesampling interval multiplied by 2 raised to the power N of the number inthe GENCNT register. For computing the received signal average, thenumber five is stored in the GENCNT register, and the time constant issixty-four milliseconds.

The decision process which determines whether the receiver issynchronized and locked with the recurrence rate of the pulse-modulatedsignal received from the transmitter is based on the relative timingbetween each two millisecond sample interval and the recurrence of eachpulse leading edge and duration.

No decisions are made affecting safety on a single sample. The purposeof the LKSYNC routine is to determine that synchronization has beenestablished with the incoming modulation signal as indicated by thecount in the register LKCNTX. The register LKCNTX is incremented only ifthree criteria are met: (1) the bit pattern in the least significantbits of the DATAX register is a predetermined bit pattern ("0-0-1") andthe SYNCX register is zero (BSYNC); (2) the first sample following the"0-0-1" bit pattern is a one (ONETS); and (3) the fourth samplefollowing the "0-0-1" bit pattern is a zero (ZROTS). Failure of thesecond or third item will result in decrementing the LKCNTX register.Also the failure to receive another "0-0-1" bit pattern prior to theeighth sample following a valid "0-0-1" pattern will decrement theregister LKCNTX.

Lock to the incoming modulation is declared by setting LKFLG when thecount in the LKCNTX register exceeds a predetermined number (e.g.,twenty-three). The minimum time to lock is a predetermined number (e.g.,twenty-four) of periods of the incoming modulation (about 343milliseconds in the implementation shown). Only after lock is achievedis the magnitude of the signal used to determine the arming indicator.This process is controlled in the ONETS routine by first computing aPKAVGX with an exponential average taken over sixteen samples (GENCNTequal four). When the PKAVGX exceeds the preset arming threshold theARMFLG is set. With both ARMFLG and LKFLG set the unit is essentiallyenabled and the machine tool can be activated upon request. ONETS alsocontrols the execution of the SAFTY routine by setting the microcomputerflag FO if the ARMFLG and LKFLG are set.

In the routine SAFTY the value of the latest sample is compared to thepresent value of the SAFTY level potentiometer. Two successive samplesseparated by one period of the modulation in time being greater than thepreset level will cause the indiator SFTYFLG to be set. The indicatorSFTYFLG will disable the machine tool even though the LKFLG and ARMFLGare both set. In the instance where the machine is being activated, theappearance of a SAFTY condition will generate an override function whichwill force the machine tool to a safe state.

The synchronization and lock decision processing is implemented usingthe two eight-bit data registers DATAX and SYNCX. The register DATAX isused to maintain a running record of the relative amplitude of thereceived signal at the sample intervals. The SYNCX register is used as abit position counter and synchronization status register. With referenceto the flow diagrams 16A through 16D, after a received signal is sampledand converted and the signal average computed using the CMPAVGsubroutine (FIG. 15), the subroutine LKSYNC is entered (FIG. 16A), thesystem control flag FO is set to zero and the bit pattern in the DATAXregister is shifted left one place. The lowest order bit in the DATAXregister is set to a logic one state if the sample value is equal to orgreater than the running average stored in the SIGAVX register,otherwise it is zero. The progressive filling of the DATAX register isshown in FIG. 13E for a typical detected signal (FIG. 13B). Theoccurrence of a "0-0-1" pattern in the three least significant bits ofthe DATAX register and the SYNCX register being zero are the criteriathat determine whether synchronization with the incoming modulationsignal zero-to-one transition has occurred.

The SYNCX register is used to control processing once a valid "0-0-1"pattern (a pattern that appears while the SYNCX register is zero) in theDATAX register is found. The progressive shifting of the SYNCX registeris also shown in FIG. 13E. When the SYNCX register is initally zero,execution is governed by the routine BSYNC (FIG. 16B). On branching fromLKSYNC (FIG. 16A) to BSYNC (FIG. 16B) during the third sample interval,the low order bit pattern in the DATAX register is found to be "0-0-1".The most significant bit of the SYNCX register is set to one. WhenLKSYNC is reentered during the fourth sample interval the left rotationof the SYNCX register moves the logic one to the least significant bitposition. Processing is then vectored based on the bit position of theone in the SYNCX register (FIG. 16A).

At the next entry to LKSYNC the ONETS routine (FIG. 16C) is executed totest the least significant bit of the DATAX register for a one. If alogic zero is found and not the expected logic one, the routine RESYNC(FIG. 16B) is entered. The count in a register LKCNTX is decremented(provided the count is greater than zero). If the count in the registerLKCNTX is less than sixteen (an arbitrary value) the system flags LKFLGand ARMFLG are set to zero. However, if the least significant bit in theDATAX register should be a logic one, a peak signal average is computedby the subroutine CMPAVG (FIG. 15) in the same manner as for the runningsignal average except that a different value of GENCNT (equal to four)is used.

The routine ONETS also determines whether the safety system is armed. Ifthe system control flag LKFLG is a logic zero (the system is not"locked" to the received signal, derived as discussed herein) the armflag ARMFLG is set to a logic zero. When the system is synchronized andlocked to the received signal and the computed peak average value is atleast one-half dB above the predetermined arm value (as set by the valueon the potentiometer 354 (FIG. 8), the flag ARMFLG is set to logic one.If the peak average falls more than 1.5 dB below the ARM threshold,ARMFLG is reset to zero.

The fifth and sixth sample intervals result in the successive shiftingof the logic one from the least significant bit position to the BIT-2position in the SYNCX register. The routine LKSYNC (FIG. 16A) branchesto the routine BSYNC during each of these intervals because the SYNCXpattern is either not "1-0-0-0-0-0-0-0" or it is in a state which wouldselect ONETS (FIG. 16C) or ZROTS (FIG. 16D) during these intervals.

In the seventh sample interval, BIT-3 position of the SYNCX registerbecomes logic one and processing branches to the routine ZROTS (FIG.16D). On entry to the routine ZROTS, the least significant bit of theDATAX register is tested. If this bit is not a logic zero, processingbranches to the routine RESYNC (FIG. 16B) to decrement the count in theregister LKCNTX and begin a search for a "0-0-1" synchronizing bitpattern in the DATAX register by setting the SYNCX register to zero. Ifthe least significant bit is a logic zero, the count in the registerLKCNTX is incremented (unless it has been previously incremented to itsmaximum value of thirty-two). It should be appreciated that LKCNTX canbe incremented only once per modulation period if the received signalpasses "0-0-1" bit pattern test, the ONETS and ZROTS. When theseconditions have been met on successive modulation periods to allowLKCNTX to be incremented to at least twenty-four, the flag LKFLG is setto one. The system is then declared locked. The peak average register iscleared and a new peak average started after "lock" is acquired toimprove system noise immunity.

The SAFETY LEVEL condition is tested on each RETURN to the main programfrom the subroutine LKSYNC and specifically via the SAFETY subroutine.The SAFETY subroutine is shown in FIG. 17. This subroutine is executedonly if the system control flag FO has been set to logic one by theroutine ONETS (FIG. 16C). Recall that the flag FO is set to logic oneonly after ARMFLG indicator is set to logic one and ONETS is satisfiedmeaning that is the peak of the incoming modulation waveform. Should theflag FO be set and the peak average of the received signal be less thanthe threshold set by the potentiometer 352 (FIG. 8), safety control flagindicator SFTYFLG is set to logic zero. On the other hand, if twosuccessive samples of the received signal, one period apart, are greaterthan or equal to the value set by the potentiometer 352, then SFTYFLG isset to logic one.

The microcomputer 460 also branches to a self-test routine when it isdesired to verify system operation. This routine is shown in flowdiagram form in FIG. 18 and at APP. PG. 8 through 11.

Those skilled in the art, having the benefit of the teachingshereinabove set forth may effect numerous modifications thereto. Thesemodifications are to be construed as lying within the contemplation ofthe instant invention as defined in the appended claims.

What is claimed is:
 1. A machine tool safety system comprising:anelectric field generator adapted to be carried on a portion of theperson of a machine tool operator for generating an electric field; acapacitive receptor antenna mountable in a predetermined location withrespect to a machine tool and cooperable with the electric fieldgenerator to form a capacitively coupled transmission arrangementoperative to induce, by the action of the electric field through thecapacitive coupling, a monotonically increasing electrical signal themagnitude of which is functionally related to the distance between theelectric field generator and the capacitive receptor antenna; a signalprocessor connected to the capacitive receptor antenna and responsive tothe signal induced in the capacitive receptor antenna by the electricfield to generate an indication when the magnitude of the inducedelectrical signal exceeds a predetermined threshold, the indicationbeing able to be used to permit safe operation of a machine tool; aself-test radiating antenna disposed in proximity to the capacitivereceptor antenna; and a self-test network electrically connected to theself-test radiating antenna for generating an excitation signaloperative to induce in the capacitive receptor antenna a test signalhaving a magnitude greater than the threshold.
 2. The machine toolsafety system of claim 1 whereinthe self-test network is adapted togenerate a second excitation signal operative to induce in thecapacitive receptor antenna a second test signal having a magnitudegreater than a second threshold.
 3. The machine tool safety system ofclaim 1 whereinthe self-test network is oprative to vary the excitationsignal at a predetermined radio frequency.
 4. The machine tool safetysystem of claim 1 or 3 whereinthe self-test network is operative toimpart a predetermined modulation characteristic to the excitationsignal.
 5. The machine tool safety system of claim 2 whereintheself-test network is operative to vary the excitation signals at apredetermined radio frequency.
 6. The machine tool safety system ofclaims 2 or 5 whereinthe self-test network is operative to impart apredetermined modulation characteristic to the excitation signals. 7.The machine tool safety system of claim 6 whereinthe modulationcharacteristic comprises a predetermined recurrence rate and apredetermined duty cycle.
 8. The machine tool safety system of claim 4whereinthe modulation characteristic comprises a predeterminedrecurrence rate and a predetermined duty cycle.
 9. The machine toolsafety system of claims 1, 2, 3 or 5wherein the capacitive receptorantenna comprises a conductive member configured in a predeterminedmanner to provide a sensitivity zone of a predetermined correspondingshape about the capacitive receptor antenna and wherein the self-testantenna comprises a wire conductor spaced a predetermined distance fromthe conductive member.
 10. The machine tool safety system of claims 1,2, 3 or 5 whereinthe transmitter is battery operated; and furthercomprising a network for monitoring the output voltage of the batteryand for providing an indication representative of the battery outputvoltage falling below a predetermined voltage threshold.
 11. The machinetool safety system of claim 4 whereinthe transmitter is batteryoperated; and further comprising a network for monitoring the outputvoltage of the battery and for providing an indication representative ofthe battery output voltage falling below a predetermined voltagethreshold.
 12. The machine tool safety system of claim 6 whereinthetransmitter is battery operated; and further comprising a network formonitoring the output voltage of the battery and for providing anindication representative of the battery output voltage falling below apredetermined voltage threshold.
 13. the machine tool safety system ofclaim 1 further comprisingmeans disposed within the self-test networkfor preventing the enablement of a machine tool in the absence of aninduced signal resulting from the field generated by the electric fieldgenerator.
 14. The machine tool safety system of claim 1 furthercomprisingmeans for preventing the enablement of a machine tool unlessthe signal induced by the excitation signal exceeds the first threshold.15. The machine tool safety system of claim 1 further comprisingmeansfor preventing the generation of the excitation signal so long as aninduced signal resulting from the field generated by the electric fieldgenerator is present.